Formable magnesium based wrought alloys

ABSTRACT

Formable magnesium based wrought alloys include a magnesium based wrought alloy consisting essentially of (wt %): 0.1 to 2.0 of Zn; 0.05 to 1.5 of Ca; 0.1 to 1.0 of Zr; 0 to 1.3 of a rare earth element or mixture of the same of which includes Gd or Y; 0 to 0.3 of Sr, Al: 0 to 0.7; the balance of Mg and other unavoidable impurities.

TECHNICAL FIELD

The present invention generally relates to new magnesium-calcium alloy compositions. More specifically, the present invention particularly relates to magnesium-zinc-rare earth-calcium-zirconium and magnesium-calcium-zinc-(zirconium) based wrought alloys which include a number of alloying elements to enhance formability, magnesium based wrought alloy sheets formed therefrom and a method or process of forming said magnesium based wrought alloy sheets.

BACKGROUND OF THE INVENTION

The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

Magnesium (Mg) is the lightest structure material with a density of 1.74 g/cm³ at 20° C., which is approximately ⅔ the density of the aluminium (Al) and ¼ the density of steel. This characteristic makes it as a promising candidate for the substitution of steel and Al alloys. There has been a rapidly growing interest in the development of magnesium sheet alloys for structural applications, especially in automobile, aerospace, light-rail, high-speed train industries. This is because the wide applications of magnesium alloys can support energy saving thereby reducing the running cost. However, traditional Mg sheet alloys, e.g. AZ31 and ZK60, have not been widely used in the industrial field due to their poor ductility and formability at moderate temperature.

Conventional magnesium wrought/sheet alloys, for example AZ31, ZK60, develop a strong basal texture during thermomechanical processing and have strong anisotropic mechanical properties. Therefore, these alloys exhibit formability inferior to their metallic competitors such as aluminium and steel alloys at moderate temperature. The limited formability of existing magnesium commercial alloys at moderate temperatures restricts the wider application of these materials.

The formability of Mg alloy sheets can be improved by modification of alloy compositions and control of processing parameters. Mg-RE (rare earth) alloy sheets show a substantial improvement in the ductility and formability in comparison with a commercial AZ31 alloy sheet. Moreover, according to the recently reported literature, the addition of non-RE element, i.e. Zn, to the Mg-RE alloy could deliver better ductility and formability than the Mg-RE binary alloy sheet. Nevertheless, no literature has yet report how to prepare a magnesium alloy sheet that has better ductility and formability than the Mg—Zn—Gd alloy sheet.

It would therefore be desirable to develop new magnesium alloys, preferably Mg based wrought alloys including Ca for sheet formation having good formability and mechanical properties.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a magnesium based wrought alloy consisting essentially of (wt %): 0.1 to 2.0 of Zn; 0.05 to 1.5 of Ca; 0.1 to 1.0 of Zr; 0 to 1.3 of a rare earth element or mixture of the same of which includes Gd or Y; 0 to 0.3 of Sr, Al: 0 to 0.7; the balance of Mg and other unavoidable impurities.

The present invention relates in this first aspect to magnesium-zinc-rare earth-calcium-zirconium and magnesium-calcium-zinc-(zirconium) based wrought alloys which include a number of alloying elements to enhance formability. The alloy comprises a dilute alloying composition, with the total amount of alloying elements preferably being less than (or equal to) 4 wt %. Whilst not wishing to be limited to any one theory, the inventors have found that a dilute alloying composition including the addition of low-cost alloying element Ca to Mg—Zn-RE-Zr and Mg—Zn—(Zr) alloys can remarkably weaken the texture and improve the formability of the alloys. The formability of the resulting Mg—Zn—RE-Ca—Zr alloys with appropriate compositions has a better formability than existing Mg—Zn—Gd—(Zr) alloy.

Depending on the alloy composition, an amount of a rare earth element may be present. In the most general form, the magnesium alloy includes (wt %): 0 to 1.3 of a rare earth element or mixture of the same, though in some forms the rare earth element or mixture of the same may comprise between 0.05 wt % and 1.3 wt %. The rare earth element or mixture of the same may comprise a rare earth element of the lanthanide series or yttrium. For the purposes of this specification the lanthanide elements comprise the group of elements with an atomic number including and increasing from 57 (lanthanum) to 71 (lutetium). Such elements are termed lanthanide because the lighter elements in the series are chemically similar to lanthanum. Strictly speaking lanthanum is a group 3 element and the ion La³⁺ has no f electrons. However for the purposes of this specification lanthanum should be understood to be included as one of the rare earth elements of the lanthanide series. For present purposes, yttrium will also be considered to be encompassed by the term “rare earth element”. Therefore the rare earth elements of the lanthanide series comprise: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. In some embodiments, the rare earth component comprises gadolinium (Gd). In other embodiments, the rare earth component comprises a mixture of gadolinium (Gd) and lanthanum (La). In other embodiments, the rare earth component comprises a mixture of gadolinium and yttrium. In other embodiments, the rare earth component comprises a mixture of gadolinium or yttrium and a rare earth element of the lanthanide series. An advantage of an embodiment comprising a rare earth element of the lanthanide series or yttrium is their relatively high solubility in magnesium.

It should be appreciated that the alloying elements function as follows: Rare earth elements are added to weaken texture and thus improve the formability of Mg—Zn alloys at moderate temperature. Zirconium is added as a grain refiner. Aluminium is added for accelerating the age hardening response of Mg—Ca—Zn—(Zr) based alloys.

A second aspect of the present invention provides a magnesium based wrought alloy consisting essentially of (wt %): Zn: 0.1 to 2.0; Ca: 0.05 to 1.5; Zr: 0.1 to 1.0; Gd: 0. to 1.0, preferably 0.05 to 1.0; Sr: 0 to 0.3; La: 0 to 0.3; Al: 0 to 0.7; and the balance of Mg and other unavoidable impurities, wherein the total weight % of alloying elements is less than 4%.

In preferred embodiments, the magnesium based wrought alloy consists essentially of (wt %): Ca: 0.3 to 1.0; Zn: 0.3 to 1.0; Zr: 0.2 to 0.7; Gd: 0.1 to 0.5; Sr: 0 to 0.2; La: 0 to 0.2; Al: 0 to 0.5; and the balance of Mg and other unavoidable impurities, wherein the total weight % of alloying elements is less than 4%.

In some embodiments, the present invention can be divided into two general calcium containing magnesium wrought alloys compositions which include several alloying elements to enhance their formability alloy compositions. The general alloy groups are as follows:

Group 1: Mg—Zn—Gd—Ca—Zr based alloys; and

Group 2: Mg—Ca—Zn—(Zr) based alloys.

The specific alloy compositions of these individual groups will now be discussed:

Group 1: Mg—Zn—Gd—Ca—Zr Based Alloys.

In group 1, Mg alloys include more than 0.5% but less than 2.0% of Zn, 0.05% to 1.0% of Gd, 0.05% to 1.0% of Ca, 0.1% to 1.0% of Zr, 0% to 0.3% strontium (Sr), 0% to 0.3% lanthanum (La), 0% to 0.7% Al and the balance of Mg, and other unavoidable impurities. In addition, preferably, the amount of Zn is ranging from 0.5% to 1.5%. Furthermore, the amount of Gd is preferably greater than 0.1% and less than 0.5%. In addition, it is preferable to contain greater than 0.1% and less than 0.7% of Ca. Furthermore, the amount of Zr is preferably greater than 0.2% and less than 0.7%. In addition, the amount of Sr is preferably less than 0.2%. Furthermore, it is preferable that the content of La is less than 0.2%. In addition, the amount of Al is preferably greater than 0.2% less than 0.5%.

Group 2: Mg—Ca—Zn—(Zr) Based Alloys.

In group 2, Mg alloys include greater than 0.3% but less than 1.5% of Ca, 0.1% to 0.8% of Zn, 0 to 1.0%, preferably 0.1% to 1.0% of Gd, 0% to 0.7% of Al, 0% to 0.3% Sr, 0.1% to 1.0% of Zr, and the balance of Mg, and other unavoidable impurities. In addition, it is preferable that the content of Ca is ranging from 0.6% to 1.0%. Furthermore, the amount of Zn is preferably greater than 0.3% and less than 0.5%. In addition, the amount of Gd is preferably greater than 0.1% and less than 0.5%. Furthermore, the amount of Al is preferably greater than 0.1% and less than 0.7%, more preferably the amount of Al is greater than 0.2% and less than 0.5%. In addition, the amount of Sr is preferably less than 0.2%. Furthermore, the amount of Zr is preferably greater than 0.2% and less than 0.7%.

The total amounts of alloying elements is preferably less than 4%, more preferably less than 3%, and yet more preferably less than 2.5%. It should be appreciated that further alloying addition can be harmful to the formability of Mg wrought alloys as it leads to formation of second phase particles that may act as nucleation sites for cracks during deformation.

In embodiments, the magnesium based wrought alloy is selected from one of: Mg-1Zn-0.4Gd-0.5Zr, Mg-1Zn-0.4Gd-0.2Ca-0.5Zr, Mg-1Zn-0.4Gd-0.5Ca-0.5Zr, Mg-1Zn-0.4Gd-0.2Ca-0.1Sr-0.5Zr, Mg-1Zn-0.4Gd-0.2Ca-0.1La-0.5Zr, Mg-0.8Ca-0.4Zn, Mg-0.8Ca-0.4Zn-0.4Gd, Mg-0.8Ca-0.4Zn-0.3Al, Mg-0.8Ca-0.4Zn-0.3Al-0.1Sr, Mg-0.8Ca-0.4Zn-0.5Zr, Mg-0.8Ca-0.4Zn-0.1Sr-0.5Zr, Mg-0.8Ca-0.4Zn-0.4Gd-0.5Zr, or Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr.

In preferred embodiments, the magnesium based wrought alloy is selected from one of: Mg-1Zn-0.4Gd-0.2Ca-0.5Zr or Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr.

Manganese (Mn) can be also added to both Zr-free and Zr-containing alloys to minimise the content of iron and to further improve corrosion resistance. If present, the amount Mn is preferably greater than 0.05% and less than 0.7%, more preferably greater than 0.1% and less than 0.5%.

The magnesium based alloy preferably comprises a minimal amount of incidental impurities. In some embodiments the magnesium based alloy comprises incidental impurities having less than having less than 0.5% by weight, more preferably less than 0.2% by weight. The incidental impurities may comprise Li, Be, Ca, Sr, Ba, Sc, Ti, Hf, Mn, Fe, Cu, Ag, Ni, Cd, Al, S Ge, Sn, and Th, alone, or in combination, in varying amounts.

The present invention also relates to a magnesium based wrought alloy sheet comprising at least one magnesium based wrought alloy according to the first or second aspects of the present invention. In this respect, the inventors are not aware of the application of Mg—Ca—Zn—(Zr) alloy system in sheet form in reported literature or in patent publications. The inventors therefore consider the sheet form of the magnesium based wrought alloy to be unique.

The present invention also relates to a method of fabricating a magnesium based alloy sheet product. A third aspect of the present invention therefore provides a method of fabricating a magnesium based alloy sheet product, the method comprising:

providing a magnesium alloy melt from the magnesium based alloy according to the first or second aspect of the present invention;

casting said magnesium alloy melt into a slab or a strip according to a predetermined thickness;

homogenising or preheating said cast slab or strip;

successively hot rolling said homogenised or preheated slab or strip at a suitable temperature to reduce said thickness of said homogenised slab or strip to produce an alloy sheet product of a predetermined thickness; and

annealing said alloy sheet product at a suitable temperature for a period of time.

The magnesium alloy melt can be produced using any suitable method. In many embodiments, the respective elements were mixed and melted in a furnace, for example a high frequency induction melting furnace, in a suitable receptacle, such as a mild steel crucible to a temperature above the liquidus temperature for that alloy embodiment. In some embodiments, the melt is heated to approximately 760° C. under an argon atmosphere.

The casting step can comprise any suitable casting process. For example, the casting step may involve casting an ingot or billet. In other embodiments, the casting step may involve casting into sheet or strip. In some embodiments, casting comprises pouring the magnesium alloy melt into one of a direct chill (DC) caster, a sand caster, or a permanent mould caster. For example, the casting step may include using a DC cast billet which is subsequently extruded to form a slab or strip after preheating. In other embodiments, the casting step comprises feeding the magnesium alloy melt between rolls of a twin-roll caster to create a strip.

The homogenising or preheating of the cast slab or strip preferably occurs at a temperature of between 300 to 500° C. The actual homogenising temperature is dependent upon alloy composition. In some embodiments, the homogenising or preheating of the cast slab or strip is followed by quenching, preferably water quenching. The homogenising or preheating of the cast slab or strip is preferably carried out for a period of about 0 to 24 hours.

The homogenised slab or strip are preferably machined into strips of 5 mm thickness and then hot rolled. Hot rolling is preferably conducted in the temperature range of 300 to 550° C., more preferably 350 to 500° C. Hot rolling typically results in a total thickness reduction of 50 to 95%, preferably 70 to 80%.

In some embodiments, hot rolling is conducted using a plurality of rolling passes, in which after each rolling pass, the sheets were reheated at a temperature in the range of 350 to 500° C. prior to subsequent rolling. The sheets are preferably reheated for about 5 to 20 minutes, preferably 5 to 10 minutes. The thickness reduction per pass is preferably about 20%. Accordingly, the total reduction can be about 80% with the thickness reduction per pass being about 20%.

After the final rolling, the sheets are given a final annealing treatment to remove accumulated strains through static recrystallization. The annealing temperature preferably is ±50° C. from the inflection point of an annealing curve obtained for a composition of the alloy for a standard period of 1 hour. Furthermore, the period of time to anneal said alloy sheet product is preferably approximately 1 minute to 24 hours.

The inventors have found that it is possible to strengthen various embodiments of the magnesium-calcium-zinc-(zirconium) based alloys of the present invention by an artificial ageing treatment. Therefore in some embodiments, the method can further comprise subjecting the annealed alloy to an age hardening treatment comprising heating the alloy at 150° C. for at least 1 hour. The age hardening period depends on the requisite or sufficient period of time to obtain the maximum precipitation hardening.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the figures of the accompanying drawings, which illustrate particular preferred embodiments of the present invention, wherein:

FIG. 1 is a flow chart of depicting a method of fabricating magnesium wrought alloys in accordance with invention including experimental testing regime.

FIG. 2 provides tensile stress-strain curves of as-annealed of B1, B2, B3, B4, and B5 alloy sheets and comparative as-annealed AZ31 and T4-6016 Al alloy sheets.

FIG. 3 provides tensile stress-strain curves of as-annealed of B6, B7, B8, B9, B10, B11, B12, and B13 alloy sheets and comparative as-annealed AZ31 and T4-6016 Al alloy sheets.

FIG. 4 provides a plot of the age hardening response of the Mg—Ca—Zn—(Zr) based alloy sheets.

FIG. 5 provides a plot of the variation of hardness of the B2 alloy sheet as a function of annealing time at temperature of 350, 400, 450 and 500° C.

DETAILED DESCRIPTION

The present invention relates to magnesium-zinc-rare earth-calcium-zirconium and magnesium-calcium-zinc-(zirconium) based wrought alloys which include a number of alloying elements to enhance formability. The present invention reveals that the formability of magnesium-zinc-rare earth-calcium-zirconium and magnesium-calcium-zinc-(zirconium) based wrought alloys such as Mg—Zn—Gd—Zr alloys are enhanced with the addition of trace amount or dilute alloying amount of Ca.

FIG. 1 illustrates a flow chart depicting a method of fabricating a magnesium alloy sheet of the present invention. As shown in FIG. 1, a magnesium-zinc-rare earth-calcium-zirconium and magnesium-calcium-zinc-(zirconium) based wrought alloy according to the composition described herein are first provided in the initial step 105.

Following melt preparation, the respective alloys are cast using a suitable casting technique in step 110. In some embodiments, the casting step may involve casting an ingot, billet, bar, block or other moulded body. In other embodiments, the casting step may involve casting into a sheet or strip.

Examples of casting techniques include twin roll casting (TRC), sand casting with or without chill plates on the two faces of the casting or DC casting. It should be appreciated that a number of direct chill (DC) casting methods and apparatus suitable for magnesium alloys are well known in the art and can be used in the process/method of the present invention. The strip or slab could also be made from a DC cast billet which has been subsequently extruded to a slab or strip again using methods and apparatus suitable for magnesium alloys that are well known in the art.

In one embodiment, alloys were melted and cast using a high frequency induction melting furnace using a mild steel crucible at approximately 760° C. under an argon atmosphere. The resulting melt was cast into suitably sized ingots 30 mm thick×55 mm width×120 mm length.

Homogenisation or preheating is employed to reduce the interdendritic segregation and compositional differences associated with the casting process. A suitable commercial practice is to choose a temperature, usually 5 to 10° C., below the non-equilibrium solidus. Given that magnesium, calcium and zinc are the major constituents in the alloys, a temperature range of 300 to 500° C., depending upon alloy composition. The time required for the homogenisation step is dictated by the size of the cast ingot, billet, strip or slab. For TRC strip a time of 2 to 4 hrs is sufficient, while for sand cast slab or direct-chill cast slab up to 24 hrs will be required. The homogenisation treatment is followed by a quenching step, typically a water quenching step.

For experimental purposes, the homogenised ingots are machined into strips of 5 mm thickness. However, it should be appreciated that strips can be formed using any number of other techniques as discussed above in the casting step.

The homogenised ingots, strips or slabs are then hot rolled at a suitable temperature, in step 120. Depending on the cast material different rolling steps may be used. For alloy slabs with a thickness above 25 mm produced by sand casting, DC casting or any other type of casting, a break-down rolling step can be used. The aim of this step is to reduce the thickness, as well as to refine and remove the cast structure. The temperature for this step is dependent on the furnace available at the rolling facility, but usually a temperature between 350 to 500° C. is employed. For alloy strips produced by TRC, rolling is performed at a temperature between 250° C. to 450° C. without the need of a break-down rolling step. Hot rolling involves the strip to pass between the rollers a number of times. After each rolling pass, the sheets are typically reheated at a temperature in the range of 350 to 500° C. for about 5 to 10 minutes prior to subsequent rolling to bring the temperature up before the next pass. A few cold passes with a percentage reduction per pass of 10% may also be used as a final rolling or sizing operation. This process is continued until the final thickness (within the set tolerances) is achieved, at step 125. The total reduction can be about 80% with the thickness reduction per pass being about 20%.

After the final rolling, the sheets were given an annealing treatment at a suitable temperature and time to remove accumulated strains through static recrystallization in step 130. Annealing is a heat treatment process designed to restore the ductility to an alloy that has been severely strain-hardened by rolling. There are three stages to an annealing heat treatment—recovery, re-crystallisation and grain growth. During recovery the physical properties of the alloy like electrical conductivity is restored, while during recrystallisation the cold worked structure is replaced by new set of strain-free grains. Recrystallisation can be recognised by metallographic methods and confirmed by a decrease in hardness or strength and an increase in ductility. Grain growth will occur if the new strain-free grains are heated at a temperature above that required for recrystallisation resulting in significant reduction in strength and should be avoided. Recrystallisation temperature is dependent on the alloy composition, initial grain size and amount of prior deformation among others; hence, it is not a fixed temperature. For practical purposes, it may be defined as the temperature at which a highly strain-hardened (cold worked) alloy recrystallises completely in 1 hour.

The optimum annealing temperature for each alloy and condition is identified by measuring the hardness after exposing the alloy at different temperatures up to 1 hr, and establishing an annealing curve to identify the approximate temperature at which re-crystallisation ends and grain growth begins. This temperature may also be identified as the inflection point of the hardness-annealing temperature curve. This method allows achieving the optimum temperature easily and reasonably accurately.

Thereafter, the annealed strips were quenched in a suitable medium, for example water.

EXAMPLES

A series of experiments were undertaken to test the relative merit of the described alloy embodiments, and to establish the low temperature formability of the alloys having been fabricated to form a sheet product.

A number of alloy compositions including alloys developed according to the present invention (B1 to B13) and comparative samples (AZ31 and T4-Al 6016) were formed and tested in these experiments. Table 1 summarises the composition of each of the tested alloy compositions.

TABLE 1 The composition of each of the tested alloy compositions. Designation Nominal composition (wt. %) B1 Mg—1Zn—0.4Gd—0.5Zr B2 Mg—1Zn—0.4Gd—0.2Ca—0.5Zr B3 Mg—1Zn—0.4Gd—0.5Ca—0.5Zr B4 Mg—1Zn—0.4Gd—0.2Ca—0.1Sr—0.5Zr B5 Mg—1Zn—0.4Gd—0.2Ca—0.1La—0.5Zr B6 Mg—0.8Ca—0.4Zn B7 Mg—0.8Ca—0.4Zn—0.4Gd B8 Mg—0.8Ca—0.4Zn—0.3Al B9 Mg—0.8Ca—0.4Zn—0.3Al—0.1Sr B10 Mg—0.8Ca—0.4Zn—0.5Zr B11 Mg—0.8Ca—0.4Zn—0.1Sr—0.5Zr B12 Mg—0.8Ca—0.4Zn—0.4Gd—0.5Zr B13 Mg—0.8Ca—0.4Zn—0.1Sr—0.4Gd—0.5Zr AZ31 Mg—3Al—1Zn—0.3Mn T4-Al 6016 Al—1.3Si—0.25Fe—0.11Mn—0.4Mg

A sheet of each of the alloy compositions were produced using the above described method. In these experiments, respective elements were mixed and melted in a high frequency induction melting furnace using a mild steel crucible at approximately 760° C. under an argon atmosphere. The homogenisation treatments were done in the temperature in a range of 300 to 500° C., depending upon alloy composition. The homogenisation treatment is followed by a water quenching step. The homogenised ingots were machined into strips of 5 mm thickness and then hot rolled in the temperature range of 350 to 500° C. The total reduction was about 80% with the thickness reduction per pass being about 20%. After each rolling pass, the sheets were reheated at a temperature in the range of 350 to 500° C. for about 5 to 10 minutes prior to subsequent rolling. After the final rolling, the sheets were given an annealing treatment to remove accumulated strains through static recrystallization.

These sheets were then subjected to mechanical testing, as described in the following examples:

Example 1: Mechanical Properties at Room Temperature

Tensile properties and formability of the as-annealed sheets of developed alloys (B1 to B16) and control samples (AZ31 and T4-Al 6016) were evaluated at room temperature.

The as-annealed sheets of each of the studied alloy compositions (see table 1) were tested along the rolling direction at a strain rate of 10⁻³/s, using a screw-driven Instron 4505 machine at room temperature. A thickness of each tensile sample was about 1 mm and gage length was about 10 mm. The samples were further rolled into 0.5 mm thickness in order to evaluate the room temperature formability of developed alloys by the mini deep drawing test with a 6 mm diameter punch. The diameters of the annealed disks were 9, 9.5, 10, 10.5, 11.5, 13.1 and 14.6 mm. The limit drawing ratio (LDR) is defined as the ratio of the largest disk diameter, which can be fully drawn without failure, to the punch diameter. To conclude, a high LDR value represents a better formability, and a low LDR value indicates a poor formability.

Table 2 summarizes mechanical properties of developed alloy sheets (B1 to B13) and comparative alloys (AZ31 and Al 6016) at room temperature. The resulting tensile stress-strain curves of the as-annealed Mg—Zn—Gd—Ca—Zr system which include the B1, B2, B3, B4, and B5 alloy sheets and comparison or benchmarks of AZ31 and Al 6016 alloys sheets are shown in FIG. 2.

The Mg—Zn—Gd—Ca—Zr based alloy sheets displayed distinctively higher ductility compared with the ductility of the AZ31 alloy sheet. Total elongation and LDR value of the B1 alloy sheet reached about 32% and 1.93, respectively. It was found that the addition of 0.2% Ca to the B1 alloy could further improve the total elongation from 32% to 38%, increase the strength from 141 MPa to 152 MPa, and enhance LDR value from 1.93 to 2.02. It is worth noting that the formability, ductility, and strength of the B2 alloy sheet are even better than that of the 6016 alloy sheet. When increasing the Ca content from 0.2% to 0.5% (B3 alloy), the ductility of the B3 alloy sheet decreased to 33%, and the LDR reduced to 1.87. The results indicates that, the Ca is essential to an improvement of the formability and strength of Mg—Zn—Gd—Ca—Zr alloys though, a strict control of the Ca content is necessary, otherwise it will produce the opposite results. Similarly, further addition of 0.1% Sr or 0.1% La to the B2 alloy would lead to an increase in the strength, but it reduced the ductility and formability.

Previously, Mg—Ca based alloys were considered to be brittle and therefore not regarded as suitable candidates for fabricating sheets. However, in the present study it has been found that a dilute addition of Zn (0.4%) to the Mg-0.8Ca alloy can greatly improve the rollability, ductility as well as formability, making the Mg-0.8Ca-0.4Zn based alloy sheets ideal for a number of industrial applications.

In these examples, alloy sheets formed from eight different Mg-0.8Ca-0.4Zn based alloys (B6 to B13) were examined. The mechanical properties of B6 to B13 are provided in table 2. In a comprehensive view, the B13 (Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr) delivered best mechanical properties in terms of ductility (23%), formability (1.83 LDR) as well as yield strength (137 MPa) among all the Mg—Ca—Zn—(Zr) based alloy sheets.

The above mechanical testing results indicate that Mg—Zn—Gd—Ca—Zr, and Mg—Ca—Zn—(Zr) based alloy sheets have higher ductility and formability than the AZ31 alloy sheet. In particular, the ductility, formability and strength of Mg—Zn—Gd—Ca—Zr based alloy sheets could even challenge that of the Al 6016 alloy, making these alloys ideal for a broad commercial application.

TABLE 2 Summarized mechanical properties of as-annealed samples of invented alloy sheets. As-annealed AZ31 and T4 treated 6016Al alloy sheets are given as benchmarks. Yield strength Designation (MPa) UTS (MPa) UE (%) TE (%) SHE LDR B1 141 213 15 32 0.19 1.93 B2 152 224 20 38 0.22 2.02 B3 155 225 21 33 0.23 1.87 B4 161 230 19 36 0.21 1.90 B5 160 229 18 26 0.22 1.78 B6 128 207 13 26 0.20 1.73 B7 125 202 16 25 0.20 1.78 B8 123 206 17 20 0.22 1.72 B9 128 207 16 27 0.21 1.70 B10 137 209 15 26 0.19 1.74 B11 132 206 17 21 0.21 1.79 B12 129 209 17 23 0.21 1.73 B13 137 215 18 23 0.21 1.83 AZ31 119 230 20 23 0.27 1.60 T4-6016 Al 153 254 20 29 0.20 1.90 NOTE: The best alloy compositions are highlighted in bold

Example 2

The annealed Mg—Ca—Zn—(Zr) based alloy sheets were subjected to an age hardening treatment by heating at 150° C. in silicone oil for a sufficient period of time to obtain the maximum precipitation hardening.

The ageing responses were measured by Vickers hardness and tensile tests. The tensile properties and formability of the age hardening sheets were evaluated at room temperature.

The ageing curves of the Mg—Ca—Zn—(Zr) based alloy sheets, with an ageing temperature of 150° C. after solution treatment (400° C. for 0.5 h), are plotted in FIG. 4.

It was found that the Mg—Ca—Zn—(Zr) based alloy sheets not only have good ductility and formability, but also possess an age hardening response characteristic. In other words, after sheets fabrication, the strength of these alloy sheets can be further improved by an ageing treatment at 150° C. As shown in the FIG. 3 and Table 3, the hardness value of the B6 alloy sheet before aging was 46 VHN. However, after the ageing treatment at 150° C. for 30 hs, the B6 alloy sheet reached the peak ageing with hardness increment of 12 VHN to 58 VHN. On the other hand, the time to reach the peak ageing extended into 72 hours when adding 0.4% Gd to the B6 alloy but shortened into 12 hours when adding 0.3% Al to the B6 alloy. Dilute addition of Gd or Al to the B6 alloy only changed the time to reach the peak aging, but the peak hardness value remained the same. Moreover, the peak hardness and required time for heat treatment would not be changed apparently when adding 0.5% Zr or 0.1% Sr to the B6 alloy.

The summary of the tensile properties of the Mg—Ca—Zn—(Zr) based alloy sheets at peak aging condition is provided in Table 4. The T6 treatment (solution treatment followed by artificial ageing) increased the yield strength of the B6 alloy sheet from 128 MPa to 153 MPa, B7 alloy sheet from 125 MPa to 146 MPa, B8 alloy sheet from 123 MPa to 163 MPa, B9 alloy sheet from 128 MPa to 164 MPa, B10 alloy sheet from 137 MPa to 166 MPa, B11 alloy sheet from 132 MPa to 174 MPa, B12 alloy sheet from 129 MPa to 166 MPa and B13 alloy sheet from 137 MPa to 168 MPa, respectively. The UE, TE and SHE of the Mg—Ca—Zn—(Zr) based wrought alloy sheets decreased, as expected. In this regard, the ductility decreased slightly, to take the B13 alloy sheet for example, the ductility of the B13 alloy sheet decreased from 23% (annealed state) to 19% (peak aged state).

TABLE 3 Initial hardness, maximum hardness, increment of hardness due to precipitation hardening and time to reach the peak hardness for the Mg—Ca— Zn—(Zr) based alloy sheets. Initial Maximum Time to reach peak Designation hardness (VHN) hardness (VHN) hardness (h) B6-T6 46 58 30 B7-T6 46 57 72 B8-T6 47 56 12 B9-T6 47 55 12 B10-T6 46 56 30 B11-T6 47 58 30 B12-T6 47 57 30 B13-T6 48 58 30

TABLE 4 Tensile properties of Mg—Ca—Zn—(Zr) based alloy sheets in peak-aged condition. Yield Designation strength (MPa) UTS (MPa) UE (%) TE (%) SHE B6-T6 153 232 13 18 0.16 B7-T6 146 228 14 20 0.17 B8-T6 163 234 13 19 0.16 B9-T6 164 233 13 19 0.16 B10-T6 166 235 13 18 0.16 B11-T6 174 241 11 17 0.15 B12-T6 166 234 14 20 0.18 B13-T6 168 236 13 19 0.16

Example 3

The previous results indicate that the B2 alloy sheet shows the best ductility and formability among the Mg—Zn—Gd—Ca—Zr systems. Accordingly, in order to further improve the mechanical properties of this alloy sheets, two critical process parameters were adjusted which would influence the performance of the sheets: rolling temperature and annealing condition.

The B2 alloy sheet was subjected to various hot rolling and annealing conditions to determine thermomechanical processing parameters for optimized mechanical properties using the previously described methodology and experimental equipment. Tables 5 and 6 summarize tensile properties of as-annealed sheet of these two alloys prepared under different thermomechanical processing conditions.

During the annealing process, recrystallisation could refine the grain size, eliminate the defects generated by the plastic deformation, and weaken the texture. Therefore, the ductility and formability of the annealed sheet increased dramatically in comparison with the as-rolled one. The experimental results show that recrystallisation could occur in the temperature range of 350° C. to 500° C. With a given annealing temperature, recrystallisation would complete when the hardness no longer decrease apparently with the extension of the annealing time.

The B2 alloy sheet was selected to optimize the annealing conditions as the final mechanical properties were closely related to the annealing temperature and time. The optimized annealing conditions for this alloy sheet were identified by measuring the variation of hardness after exposing to different temperatures for different times. Thus, in order to find the completion time of recrystallisation at different annealing temperature, hardness testing for the B2 alloy sheet sample with a different annealing temperature and annealing time.

The hardness variation as a function of expose time for the B2 sheets, which were rolled at 450° C. and subsequently annealed at different temperatures, is shown in FIG. 5. It was found that the hardness value decreased rapidly in the first one minute annealing and became steady after 1 h at 350° C., 0.5 h at 400° C., 0.5 h at 450° C., or 0.5 h at 500° C. for the B2 wrought alloy sheet. Therefore, the sheet was annealed at 350° C. for 1 h, 400° C. for 0.5 h, 450° C. for 0.5 h, and 500° C. for 0.5 h and then tested along the rolling direction. With increasing the annealing temperature, the yield strength of the B2 alloy sheet decreased as expected, while the strain-hardening exponent of these alloys slightly increased. It was noted that the UE and SHE of the B2 alloy sheet decreased after 0.5 h annealing at 500° C.

Furthermore, as shown in the table 5, the recrystallisation time of the B2 alloy sheet is 1 h when the annealing temperature is 350° C. However, when the annealing temperature is 400, 450 and 500° C., the recrystallisation time of B2 alloy sheet is 0.5 h.

Room temperature tensile test were also conducted for the B2 alloy sheet which was annealed under different conditions at 350° C. for 1 h, at 400° C., 450° C. and 500° C. for 0.5 h. The mechanical properties, including yield strength, UTS, UE, TE, and SHE of the B2 alloy sheets under different annealing conditions are summarized in Table 6. With regarding to the B2 alloy sheet, annealing treatment at 400° C. for 0.5 h that delivered a ductility of 38% was proven to be the optimal annealing condition.

TABLE 5 Summary of tensile properties of the as-annealed B2 alloy sheets rolled at different temperatures. Hot Rolling Yield Temperature strength UE TE Designation (° C.) (MPa) UTS (MPa) (%) (%) SHE B2 400 156 228 19 33 0.21 450 152 224 20 38 0.22 500 130 211 17 23 0.22

TABLE 6 Summary of tensile properties of the as-annealed B2 alloy sheets which were annealed under different conditions. Yield Annealing strength UTS TE Designation Condition (MPa) (MPa) UE (%) (%) SHE B2 350° C. 1 h 162 228 18 25 0.17 400° C. 0.5 h 152 224 20 38 0.22 450° C. 0.5 h 137 222 21 36 0.24 500° C. 0.5 h 123 219 19 30 0.24

Taking into account of the values of UE and SHE, 450° C. hot rolling and 0.5 h at 400° C. annealing delivered the best formability for both alloys.

CONCLUSION

Overall, it can be concluded from the experimental results that Mg—Ca—Zn—(Zr) based alloys show moderate formability, but they can be significantly strengthened by an artificial ageing treatment. For example, the yield strength of Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr alloy in the as-annealed state is only about 137 MPa, but it can be increased up to 168 MPa by the application of 150° C. ageing treatment.

The inventors have also found that sheets formed from Mg—Zn—Gd—Ca—Zr based alloys show superior mechanical properties in terms of strength and formability. It was found that the addition of 0.2% Ca to Mg-1Zn-0.4Gd-0.5Zr alloy led to a remarkable increase in formability (1.93 LDR to 2.02 LDR) and strength (141 MPa to 152 MPa) of the resultingly formed alloy sheet. Increasing the Ca content to 0.5% caused a slight increase in the yield strength (yield strength) from 152 MPa to 155 MPa, but the LDR value decreased from 2.02 to 1.87, compare with the Mg-1Zn-0.4Gd-0.2Ca-0.5Zr alloy. The results indicated that, while the Ca element was essential to the improvement of the formability and strength of Mg—Zn—Gd—Ca—Zr alloy sheets, a strict control of the Ca content is necessary. A further addition of 0.1% Sr or 0.1% La to the Mg-1Zn-0.4Gd-0.2Ca-0.5Zr alloy led to an increase in the strength, but a decrease in the LDR value.

In addition to the Mg—Zn—Gd—Ca—Zr alloy, the Mg—Ca—Zn—Sr—(Gd)—Zr alloy sheets also exhibited adequate strength and formability at room temperature. The Mg-0.8Ca-0.4Zn-0.1Sr-0.4Gd-0.5Zr composition showed best mechanical properties in terms of yield strength (137 MPa) and formability (1.83 LDR).

In summary, the present invention regards to the development of magnesium alloy and the resulting highly formable magnesium alloy sheets. The addition of alloying element Ca to Mg—Zn-RE-Zr alloys can remarkably improve the ductility and formability of the respective alloy sheet. More particularly, the addition of small amount of Ca to Mg—Zn—Gd—Zr based alloys results in new Mg alloy sheets that have high ductility, formability, and reasonably good strength. In this respect, (1) the addition of dilute calcium to the Mg—Zn—Gd—Ca—Zr system could substantially improve the ductility and formability; (2) the Mg—Ca—Zn—(Zr) based alloy also exhibited good ductility and formability by adding small amount of alloying elements. The ductility and formability of sheets formed from these new alloys are far better than AZ31 that have been currently used and can be comparable with 6016 alloy sheet, indicating that the alloys and corresponding alloy sheets thereof are suitable for a number of industrial applications.

The inventors consider that the above features make the inventive alloys particularly suitable for automotive applications. In addition, the inventive alloys can be processed by a range of existing manufacturing technologies, including extrusion, forging and twin-roll casting.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications may be introduced into the compositions and arrangements of steps other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.

Future patent applications may be filed in Australia or overseas on the basis of or claiming priority from the present application. It is to be understood that the following provisional claims are provided by way of example only, and are not intended to limit the scope of what may be claimed in any such future application. Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions. 

What is claimed is:
 1. A magnesium based wrought alloy consisting essentially of (wt %): 0.1 to 2.0 of Zn; 0.05 to 1.5 of Ca; 0.1 to 1.0 of Zr; 0 to 1.3 of a rare earth element or mixture of the same of which includes Gd or Y; 0 to 0.3 of Sr; 0 to 0.7 of Al, the balance of Mg and other unavoidable impurities.
 2. The alloy according to claim 1, wherein the rare earth element mixture comprises gadolinium or yttrium and a rare earth element of the lanthanide series.
 3. The alloy according to claim 1, wherein the rare earth element mixture comprises gadolinium and La.
 4. The alloy according to claim 1, wherein the rare earth element consists essentially of gadolinium.
 5. A magnesium based wrought alloy consisting essentially of (wt %): Zn: 0.1 to 2.0; Ca: 0.05 to 1.5; Zr: 0.1 to 1.0; Gd: 0 to 1.0; Sr: 0 to 0.3; La: 0 to 0.3; Al: 0 to 0.7; and the balance of Mg and other unavoidable impurities.
 6. The magnesium based wrought alloy according to claim 1, consisting essentially of (wt %): Zn: 0.3 to 1.0; Ca: 0.3 to 1.0; Zr: 0.2 to 0.7; Gd: 0.1 to 0.5; Sr: 0 to 0.2; La: 0 to 0.2; Al: 0 to 0.5; and the balance of Mg and other unavoidable impurities.
 7. The magnesium based wrought alloy according to claim 1, comprising a Mg—Zn—Gd—Ca—Zr based alloy consisting essentially of (wt %): Zn: 0.5 to 2.0; Ca: 0.05 to 1.0; Zr: 0.1 to 1.0; Gd: 0.05 to 1.0; Sr: 0 to 0.3; La: 0 to 0.3; Al: 0 to 0.7 and the balance of Mg and other unavoidable impurities.
 8. The magnesium based wrought alloy according to claim 7, comprising a Mg—Zn—Gd—Ca—Zr based alloy consisting essentially of (wt %): Zn: 0.5 to 1.5; Ca: 0.1 to 0.7; Zr: 0.2 to 0.7; Gd: 0.1 to 0.5; Sr: 0 to 0.2; La: 0 to 0.2; Al: 0.2 to 0.5 and the balance of Mg and other unavoidable impurities.
 9. The magnesium based wrought alloy according to claim 1, comprising a Mg—Ca—Zn—(Zr) based alloy consisting essentially of (wt %): Ca: 0.3 to 1.5; Zn: 0.1 to 0.8; Zr: 0.1 to 1.0; Gd: 0 to 1.0; Al: 0 to 0.7; Sr: 0 to 0.3; and the balance of Mg and other unavoidable impurities.
 10. The magnesium based wrought alloy according to claim 9, comprising a Mg—Ca—Zn—(Zr) based alloy consisting essentially of (wt %): Ca: 0.6 to 1.0; Zn: 0.3 to 0.5; Zr: 0.2 to 0.7; Gd: 0 to 0.5; Al: 0.2 to 0.5; Sr: 0 to 0.2; and the balance of Mg and other unavoidable impurities.
 11. The magnesium based wrought alloy according to claim 1, wherein the total weight % of alloying elements is less than 4%.
 12. The magnesium based wrought alloy according to claim 1, further comprising: 0.05 to 0.7 Mn.
 13. The magnesium based wrought alloy according to claim 1, wherein the magnesium based alloy comprises incidental impurities having less than 0.5% by weight.
 14. The magnesium based wrought alloy according to claim 1, wherein the magnesium based alloy comprises incidental impurities having less than 0.2% by weight.
 15. The magnesium based wrought alloy according to claim 1, selected from the group consisting of Mg-1Zn-0.4Gd-0.2Ca-0.5Zr and Mg-0.8Ca-0.4Zn-0.1 Sr-0.4Gd-0.5Zr.
 16. A magnesium based wrought alloy sheet comprising at least one magnesium based wrought alloy according to claim
 1. 17. A method of fabricating a magnesium based alloy sheet product, the method comprising: providing a magnesium alloy melt from the magnesium-based alloy according to claim 1; casting said magnesium alloy melt into a slab or a strip according to a predetermined thickness; homogenising or preheating said cast slab or strip; successively hot rolling said homogenised or preheated slab or strip at a suitable temperature to reduce said thickness of said homogenised slab or strip to produce an alloy sheet product of a predetermined thickness; and annealing said alloy sheet product at a suitable temperature for a period of time.
 18. The method of claim 17, wherein the casting comprises feeding the magnesium alloy melt between rolls of a twin-roll caster to create a strip.
 19. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the homogenising or preheating of the cast slab or strip occurs at a temperature between 300 to 500° C.
 20. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the homogenising or preheating of the cast slab or strip is followed by quenching.
 21. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein hot rolling is conducted in the temperature range of 300 to 550° C.
 22. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the hot rolling results in a total thickness reduction of 50 to 95%.
 23. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the hot rolling is conducted using a plurality of rolling passes, in which after each rolling pass, the sheets are reheated at a temperature in the range of 350 to 500° C. prior to subsequent rolling.
 24. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the casting comprises pouring the magnesium alloy melt into one of a direct chill (DC) caster, a sand caster, or a permanent mould caster.
 25. The method of fabricating a magnesium based alloy sheet product according to claim 17, further comprising subjecting the annealed alloy to an age hardening treatment comprising heating the alloy at 150° C. for at least 1 hour.
 26. The magnesium based wrought alloy according to claim 1, wherein the total weight % of alloying elements is less than 3%.
 27. The magnesium based wrought alloy according to claim 1, further comprising 0.1 to 0.5 Mn.
 28. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the homogenising or preheating of the cast slab or strip is followed by water quenching.
 29. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein hot rolling is conducted in the temperature range of 350 to 500° C.
 30. The method of fabricating a magnesium based alloy sheet product according to claim 17, wherein the hot rolling results in a total thickness reduction of 70 to 80%. 